Photoaffinity labeling is used to covalently attach selected fluorene to DNA in vitro and to macromolecular targets in vivo. This direct attachment circumvents the requirement for metabolic activation. This attachment leads to frameshift reversions in the Ames system; no mutations are observed in the dark or in strains possessing DNA repair. 2-azidofluorene has been applied to the backs of sheared Balb/c and C57/BL/6J mice. Sections of skin and viscera were taken from sacrificed animals. After two months no differences were observed among the treated mice in the dark, treated mice in the light and control animals. In vitro labeling on calf thymus DNA occurs on both purines and pyrimidines and probably on the sugar phosphate backbone also.